The invention pertains to humidity meters used for detecting humidity in various types of electronic apparatuses, electric apparatuses, toys, home electric appliances, and the like. The invention is directed, in particular, to a humidity meter formed by integrating both an oscillator circuit including a humidity sensor and peripherals thereof.
FIGS. 14A, 14B and 15 are diagrams showing a conventional example. In these drawings, reference numeral 1 denotes a humidity-frequency converting circuit; 2, a differentiating circuit; 3, a waveform shaping circuit; 4, an integrating circuit; and 5, a threshold control circuit; and reference characters R.sub.1 to R.sub.5 denote resistors; C.sub.1, C.sub.2, capacitors; G.sub.1, a gate; and Tr, a transistor.
FIG. 14A illustrates a block diagram of a conventional humidity meter; and FIG. 14B shows a waveform diagram of various parts of the humidity meter. Humidity meters have heretofore been used to detect humidity in various electronic apparatuses (e.g., copying machines, printers) and the like. One such humidity meter is shown.
As shown in FIG. 14A, this humidity meter includes: the humidity-frequency converting circuit 1; the differentiating circuit 2; the waveform shaping circuit 3; the integrating circuit 4; and the threshold control circuit 5. This humidity meter is designed to produce an output (a detected humidity) from the integrating circuit 4.
The humidity-frequency converting circuit 1 is formed by using an impedance variable humidity sensor that changes impedance exponentially in accordance with the change of humidity, and outputs a pulse train obtained by converting a change in humidity into a change in frequency.
The differentiating circuit 2 is a circuit for receiving the output pulse train from the humidity-frequency converting circuit 1 and differentiating the received pulse train to generate a pulse train whose width is narrower than that of the aforementioned pulse train. The waveform shaping circuit 3 is a circuit (threshold circuit) for extracting a differentiated wave component whose threshold level is not less than a predetermined threshold level out of the differentiated wave and shaping the waveform of the extracted wave component.
The integrating circuit 4 is designed to integrate the pulse train whose waveform has been shaped by the waveform shaping circuit 3. The threshold control circuit 5 controls the threshold level of the waveform shaping circuit 3 in response to the output voltage of the integrating circuit 4.
The operation of the humidity meter will be described below with reference to FIG. 14B. Outputted from the humidity-frequency converting circuit 1 are a pulse train whose frequency f is low on the low humidity side and a pulse train whose frequency f is high on the high humidity side (see S1). Then, these pulse trains are differentiated by the differentiating circuit 2. Each of these differentiated pulse trains (see S2) is subjected to an extracting process in which inch component whose threshold level is not less than a predetermined threshold level is extracted, and each extracted component is then subjected to a waveform shaping process by the waveform shaping circuit 3 (see S3). These waveform-shaped pulse trains are thereafter integrated by the integrating circuit 4 to obtain a humidity meter output.
Further, the output voltage of the integrating circuit 4 is applied to the threshold control circuit 5 as a control voltage, and it is with this control voltage that the threshold control circuit 5 controls the threshold level of the waveform shaping circuit 3.
The linearity of the humidity meter output is thus controlled by constantly optimizing a threshold level with respect to the magnitude of a humidity meter output in this way, and as a result, a humidity meter output with an excellent characteristic can be obtained.
FIG. 15 is an exemplary conventional humidity meter circuit. The circuit configuration of a specific example of the humidity meter is illustrated in FIG. 15.
This example is constructed by the differentiating circuit 2 of the capacitor C.sub.1 and the resistor R.sub.1 ; the waveform shaping circuit 3 of the gate (buffer gate) G.sub.1 ; the threshold control circuit 5 of the transistor Tr and the resistors R.sub.2, R.sub.3 ; and the integrating circuit 4 of the capacitor C.sub.2 and the resistor R.sub.5.
The output voltage of the capacitor C.sub.2 constituting the integrating circuit 4 is applied to the base of the transistor Tr constituting the threshold control circuit 5 through the resistor R.sub.4.
As a result of this configuration, the base current of the transistor Tr is changed by the output voltage of the integrating circuit 4, and this changes the collector-emitter resistance of the transistor Tr in accordance with the output voltage of the integrating circuit 4. As a result, the potential on the input side of the gate G.sub.1 is changed, thereby changing the threshold level.
In the aforementioned conventional humidity meter has addressed the following problems.
The aforementioned conventional humidity meter, which is of high accuracy type, requires the humidity-frequency converting circuit, the differentiating circuit, the waveform shaping circuit, the integrating circuit, the threshold control circuit, and the like. Therefore, the humidity meter formed by integrating these individual circuits is disadvantageous in that the number of parts is large and that needs for downsizing, lightening, and cost reduction are not easily met.
The aforementioned conventional humidity meter requires a large number of parts and uses expensive semiconductive elements particularly for the waveform shaping circuit and the threshold control circuit. Therefore, it is difficult to curtail the cost of manufacture, meaning that the humidity meter is expensive.
The downsizing of a humidity meter or the cost reduction of the humidity meter at the sacrifice of detection accuracy cannot be implemented by the aforementioned conventional humidity meter. That is, a humidity meter of extremely small structure or extremely low cost is difficult to implement out of the configuration of the conventional humidity meter. Therefore, it is difficult to meet the aforementioned needs.